As a material for substrates for electrical parts of information and telecommunication devices (broadcast devices, mobile radio, portable telecommunication devices etc.), radar, highspeed information processing devices and the like, ceramic has been conventionally used. Substrates made of ceramic have heat resistance and can also be used for recent information telecommunication devices having a high frequency signal band (reaching the GHz band). However, ceramic is not flexible and cannot be made thin, which limits its applicable field.
Accordingly, consideration has been made as to the use of a film made of an organic material as a substrate for electrical parts, and a film made of polyimide and a film made of polytetrafluoroethylene have been proposed. The film made of polyimide has superior heat resistance and is strong, which enables advantageous provision of a thin film. On the other hand, problems of low signal strength, delay in signal transmission and the like are of concern in the application to high frequency signals. While the film made of polytetrafluoroethylene is also compatible with high frequency, it is associated with problems in that the film cannot be made thin due to low elastic modulus, the film shows poor adhesion to a metal conductor, a resistor and the like on the surface, the film is unsuitable for the production of ultrafine wiring circuits due to its high coefficient of linear thermal expansion which causes marked dimensional changes due to temperature change, and the like. As a result, its applicable field is limited. As the situation stands, a film for a substrate, which can simultaneously achieve heat resistance, applicability to high frequency and flexibility, has not been obtained.
In addition, as a polyimide film having a high elastic modulus, a polyimide benzoxazole film made of polyimide having a benzoxazole ring in the main chain has been proposed (see patent reference 1). A print wiring board with the polyimide benzoxazole film as a dielectric layer has also been proposed (see patent reference 2, patent reference 3).
patent reference 1 JP-A-6-56992
patent reference 2 JP-A-11-504369
patent reference 3 JP-A-11-505184
As compared to conventional polyimide films, the polyimide benzoxazole film is also superior in the heat resistance. Thus, the curling developed during heat processing tends to be suppressed. However, further improvements have been desired as electronics are miniaturized and the wiring becomes highly dense. In the application to ultrahigh frequency reaching the milliwave region, moreover, use of even a substrate made of a conventionally reported polyimide benzoxazole film is inferior to the use of a substrate made of ceramic. To be precise, since the properties of the film vary markedly in response to humidity, the efficiency of high frequency signal transmission is low, and the response speed is low (poor rise of pulse signals), speeding up of the operation of circuits made of a polyimide film is not easy, and further improvements are desired.